1. Field of the Invention
This invention relates to a method for modifying a water-absorbent resin comprising a step of mixing a water-absorbent resin and an aqueous solution and a method for producing a modified water-absorbent resin. More particularly, this invention relates to a method for modification of a water-absorbent resin comprising a step of uniformly mixing a water-absorbent resin and an aqueous solution by using a mixing device of a specific construction, or by adding the aqueous solution in a specific range to the water-absorbent resin, and a mixing device optimally suitable for the method.
2. Description of the Related Art
The water-absorbent resin possesses a characteristic property of absorbing a large volume of water and is used for materials forming sanitary articles such as disposable diapers, sanitary napkins, and incontinence pads, water-retaining agents for soil, and drip sheets for foodstuffs. Particularly for application to such hygienic articles as disposable diapers, the desirability of developing a water-absorbent resin having a high absorption capacity under pressure has been requested with a view to conferring decreased thicknesses on such products. The water-absorbent resin of this sort is generally produced by undergoing such steps as formation of a hydrated gel polymer, drying, pulverization, and classification. For the purpose of imparting further exalted absorption properties, the water-absorbent resin is modified frequently by the addition of various water-soluble compounds. The surface cross-linkage of the water-absorbent resin is one conceivable approach to the modification and it necessitates a step of mixing a water-absorbent resin powder and an aqueous solution containing a surface cross-linking agent (JP-A-1992-214734).
The solid-liquid system is stirred and mixed in order to suspend solid particles and uniformly diffuse the liquid phase encircling the solid particles. When the solid is a water-absorbent resin, the solid and the liquid are not easily mixed uniformly and are liable to form aggregates because the water-absorbent resin after addition of the aqueous solution immediately absorbs the liquid.
With a view to effecting the mixture satisfactorily, JP-A-1992-214734 indicates use of a high-speed stirring type mixer having an inner surface formed substantially of material manifesting an interfacial contact angle of not less than about 60° to water and possessing a temperature of heat distortion of not lower than about 70° C. As concrete examples of the high-speed stirring type mixer, mixers of the type having rotary vanes equipped on the inner surface of the bottom part of a stirring tank such as Henschel Mixer, New Speed Mixer, and Heavy Duty Matrix and mixers of the type equipped on the inner side thereof with a cylindrical container and adapted to effect continuous mixture of two or more kinds of powder or a liquid and a powder by high-speed rotation of a rotor fitted with a multiplicity of paddles such as Turburizer and Sand Turbo may be cited. This publication has a mention that a continuous mixer possesses high productivity.
Then, as a means to perform covalent bond type surface cross-linkage on dried and finely divided hydrogel particulates, a method which obtains a product by introducing a hydroxy gel powder and a solution of ethylene glycol diglycidyl ether, water, and 1,2-propane diol as kept mixed into a plow-shaped mixing device over a period of 5-10 minutes and subsequently heating, and cooling the resultant mixture is described (Example 7 in JP-A-2002-527547). From the embodiment given in this example that the mixing operation was continued over a period of at least five minutes and the product was classified with sieves to remove particles measuring 120-850 μm in diameter, it is inferred that aggregates are generated in this example.
A method has been known which, in mixing water-absorbent resin particles and liquid drops by the use of a mixing device equipped with a stirring axis fitted with a plurality of paddles, operates the device with the linear velocity of the leading terminals of paddles set in a rang of 250-3,000 m/min (JP-A-1999-349625). In one working example cited in this publication, a surface cross-linking agent-containing solution was mixed by spraying with a water-absorbent resin by using a high-speed stirring mixing device such as Turburizer, with the rotational frequency of the Turburier set at 2,160 rpm. The mixing device, as described in the specification, was furnished with an inner surface coated with Teflon and equipped on the circumferences of paddles (vanes) with protrusions so disposed as to induce turbulence and consequently exalted the mixing property of the device.
A method has been disclosed which produces a water absorbing agent endowed with prescribed properties by mixing a carboxyl group-containing water-absorbent resin and an aqueous solution by means of a stirring continuous extrusion type mixing device equipped around a rotary shaft in the inner part of a stationary cylinder with at least one stirring member adapted to exert extruding thrust force on the water-absorbent resin (JP-A-1997-235378). Since this mixing device has the stirring member so disposed as to secure a mixing region on the discharge side of a diffusing region, the mixture of the water-absorbent resin and the aqueous solution proceeds in two or more states of agitation, namely dispersion and admixture.
A method has been disclosed which, for the purpose of enabling uniform mixture of a water-absorbent resin and a liquid substance to proceed very easily and stably for a long time, causes the water-absorbent resin and the liquid substance to be mixed by the use of a mixing device equipped with a spray nozzle (JP-A-2002-201290). This method is supposed to comprise spraying a water-absorbent resin through a spray nozzle in the shape of an empty cone describing an annular spray pattern or in the shape of an elliptic cone describing a biconvex lens spray pattern, drying the product of spraying, pulverizing the dry water-absorbent resin, and heat-treating the pulverized water-absorbent resin in an atmosphere having a dew point of not higher than 60° C. and a temperature of not lower than 90° C. As the mixing devices which are available for this method, a cylindrical mixing device, a double wall conical mixing device, a V-shaped mixing device, a ribbon type mixing device, a screw type mixing device, a fluidized type furnace rotary disc type mixing device, an air flow type mixing device, an inner mixing device, a pulverizer type kneader, a rotary type mixing device, and a screw type extruding device are cited. In the working examples, a Turburizer was used as a continuous high-speed stirring and mixing device.
For the modification of the surface of a water-absorbent resin, the step of mixing the water-absorbent resin and an aqueous solution containing a surface modifying agent indeed is an indispensable requirement. Since the water-absorbent resin, on contacting an aqueous solution, immediately absorbs the aqueous solution and swells or exerts stickiness, the aqueous solution is uniformly mixed with the water-absorbent resin only with difficulty. Even the method mentioned above, therefore, suffers occurrence of aggregates.
Further, since the water-absorbent resin swells and retains the stickiness, it adheres to the interior of the device. The aggregates occur also in consequence of the growth of the resultant deposit. Further, the occurrence of the deposit possibly renders continuous operation of the device difficult. This invention, therefore, aims to provide a mixing method which efficiently executes uniform mixture of a water-absorbent resin and an aqueous solution.
The water-absorbent resin is generally preferred to contain particulates of particle diameters of not more than 150 μm in the smallest possible amount. These particulates in such a water-absorbing product as a disposable diaper clog in the product and constitute a factor for degrading the ability of the product to pass liquid. Even when the particulates are treated with a surface cross-linking agent, the treatment does not easily attain exaltation of various solid state properties such as an absorption capacity under pressure but entails such problems as incurring loss due to the disposal of useless scrap and consequently degrading the yield and adding to the cost of waste disposal. This invention is directed toward providing a method for producing a water-absorbent resin which does not appreciably contain particulates.